German Published Patent Application No. 102 26 974 describes a method for determining the rotor position of a synchronous machine operated in field-oriented manner. Here, a sensorless position detection is implemented in that a high-frequency signal is additionally superimposed on the pulse-width-modulated output voltage of the inverter, and the high-frequency component in the associated current signal is separated from the fundamental wave component. The high-frequency component is used for determining the position angle of the rotor. The fundamental wave component is forwarded to a current controller, as actual value.
As an alternative, the high-frequency components would also be separable with the aid of a bandpass filter.
German Published Patent Application No. 10 2008 025 408 describes a method for regulating a synchronous machine, in which a rotor flux vector which is perpendicular to the induced voltage is formed and used as feedback quantity, which, when vanishing drift is present, vanishes as well.